1. Field of the Invention
The present invention relates to a positron emission tomography (PET) imaging system, and more particularly, to a low-power positron emission tomography (PET) imaging system.
2. Description of the Related Art
A positron emission tomography (PET) imaging system is an imaging system that generates an image of a structure based on the gamma rays that are emitted by the structure. PET imaging systems receive large numbers of gamma rays, detect pairs of common gamma rays from the large numbers of gamma rays, and analyze the pairs of common gamma rays to generate an image.
The pairs of common gamma rays differ from other gamma rays in that the gamma rays in each pair of common gamma rays are coincidentally-emitted from a common point of origin with directional vectors that are 180° apart from each other. Thus, by detecting the pairs of common gamma rays, the point of origin of each pair can be determined, and an image can be generated based on the point of origins of the pairs of common gamma rays.
The pairs of common gamma rays are emitted in response to positron decay. A positron is a particle with the same mass as an electron, but with a positive electric charge. Positron decay occurs when there are too many protons in a nucleus, but not enough energy to emit an alpha particle. When this occurs, a positron and an electron combine, and the two particles annihilate each other. The annihilation, in turn, creates a pair of common gamma rays that are emitted from a common point of origin with directional vectors that are 180° apart (in opposite directions) from each other.
PET imaging systems are often used in medical applications to image cellular structures within a human body. The image of a cellular structure can be enhanced by introducing glucose-like molecules, which contain atoms that exhibit positron decay, into the body. In these applications, the cellular structures of interest have high cellular activity and, therefore, use a large amount of glucose.
Since the cellular structures of interest use a large amount of glucose, the cellular structures of interest also use a large amount of the glucose-like molecules. This large use, in turn, substantially increases the number of pairs of common gamma rays that are emitted by the cellular structures of interest, thereby enhancing the image of the cellular structures of interest.
FIG. 1 shows a view that illustrates an example of a prior-art PET imaging system 100. As shown in FIG. 1, PET imaging system 100 includes a circular support structure 110, and a number of gamma ray detectors GD1-GDn that are attached to the inner surface of circular support structure 110. (Only eight gamma ray detectors GD1-GD8 are shown for purposes of clarity.) The gamma ray detectors GD1-GDn, in turn, output a corresponding number of gamma ray signals GS1-GSn, where a change in the magnitude of a gamma ray signal GS indicates the reception of a gamma ray particle. For example, a gamma ray particle can be received from a human body 112.
Conventionally, a gamma ray detector includes scintillation crystals that receive a gamma ray particle, such as from a human body, and convert the gamma ray particle into a light ray. For example, bismuth germinate (BGO), which has a high efficiency (large stopping power), and barium fluoride (BaF2), which has a faster response than BGO (although less efficient than BGO), are commonly used scintillation crystals.
In addition, a conventional gamma ray detector also includes a photo multiplier tube (PMT) that converts the light ray output by the scintillation crystals into an electric signal, and a variable gain amplifier (VGA) that amplifies the electric signal and outputs the amplified electric signal as a gamma ray signal GS.
As further shown in FIG. 1, PET imaging system 100 includes a number of analog-to-digital (A/D) converters AD1-ADn that are connected to the gamma ray detectors GD1-GDn so that each A/D converter AD1-ADn is connected to a different gamma ray detector GD to receive a different gamma ray signal GS. (Only eight A/D converters AD1-AD8 are shown for purposes of clarity. In addition, the A/D converters AD1-ADn can include input buffer amplifiers.)
In operation, the A/D converters AD1-ADn digitize the gamma ray signals GS1-GSn in response to each rising edge of a sample clock signal CLK during an image collection period of time, and output a corresponding number of digitized gamma ray signals DG1-DGn in response to the digitization. The sample clock signal CLK can have a frequency of, for example, 200 MHz.
PET imaging system 100 additionally includes a coincidence detector 114 and a gamma ray analyzer 116. Coincidence detector 114 has a number of inputs DD1-DDn that are also connected to the gamma ray detectors GD1-GDn to receive the gamma ray signals GS1-GSn so that each input DD is connected to a different gamma ray detector GD to receive a different gamma ray signal GS. In addition, gamma ray analyzer 116 has a number of analyzer inputs AA1-AAn that are connected to the A/D converters A/D1-A/Dn such that each analyzer input AA is connected to the output of a different A/D converter A/D.
In operation, coincidence detector 114 samples each of the gamma ray signals GS1-GSn in response to each rising edge of the sample clock signal CLK during the image collection period of time to identify each pair of gamma ray signals GS that represent a pair of common gamma rays. As noted above, a pair of common gamma rays has coincident emission and rays with directional vectors that are 180° apart from each other.
When coincidence detector 114 identifies a pair of gamma ray signals that represent a pair of common gamma rays, coincidence detector 114 outputs coincidence data CD to gamma ray analyzer 116. The coincidence data CD, in turn, identifies the digitized gamma ray signals DG1-DGn that correspond with the pair of gamma ray signals that represent the pair of common gamma rays.
Gamma ray analyzer 116, which is typically implemented as a digital signal processor (DSP), responds to the coincidence data CD by utilizing the digitized gamma ray signals DG1-DGn, which correspond with the pair of gamma ray signals that represent a pair of common gamma rays, to determine the origin of the pair of common gamma rays. This and other information is then used to generate an image of a cellular structure within human body 112. Gamma ray analyzer 116 also controls the gain of the VGAs in the gamma ray detectors GD1-GDn by way of feedback signal FS.
One of the drawbacks of PET imaging system 100 is that system 100 consumes a lot of power. Thus, there is a need for a PET imaging system that utilizes less energy.